1. Field of the Invention
The present invention relates to a structure for holding a decentered lens which is used in an optical system, and to a method for positioning a plurality of decentered lenses relative to each other.
2. Description of the Related Arts
Conventionally, there has been provided an axisymmetrical lens which has two spherical lens surfaces having a common axis, and there has been provided a structure for holding such a lens. The lens holding structure employs a pair of annular members each of which contacts with each lens surface within one plane, where the lens is held by the two annular members with the lens being sandwiched therebetween in the axial direction.
On the other hand, there has been proposed a decentered lens. The decentered lens is one type of lens which has:
(1) a centering axis that is not coincident with the optical axis, in case of considering a single lens which has two centers of curvature, and only one optical axis; and PA1 (2) centers of curvature that do not exist along a straight line, in case of considering a plurality of lenses which have at least four centers of curvature and more than one optical axis.
Generally, the decentered lens is not uniform in thickness around its periphery.
If one tries to apply the aforementioned conventional lens holding structure as a structure for holding the decentered lens, the decentered lens, which does not have uniform thickness around its periphery, can not be held or pinched by the annular members which contact with the lens surfaces within one planes; thus impossible to adopt the conventional lens holding structure. It is to be noted that no optical devices that positively incorporate such decentered lenses have been developed into practical use till now, and that no techniques have been established for holding the decentered lens having nonuniform thickness at its periphery, as for holding the decentered lens(es) to a lens barrel.
In other words, the non-establishment of decentered lens holding technique has been one cause that obstructs the prevalence of the decentered lenses in various kinds of optical equipments and apparatus. Free incorporation of decentered lenses into such optical equipments and apparatus, if realized, will contribute to widening the variations in designing optical systems.
The decentered lens makes an image which is distorted in a certain direction. Therefore, in case that the decentered lens is used in an optical system, the decentered lens must be held so as not to rotate around its centering axis in order that the direction in which the image is distorted is not changed. There is no need of limiting the rotation of the aforementioned axisymmetrical lens because an image formed therethrough is not distorted in any particular directions. However, it is necessary to limit the rotation of the decentered lens.
Generally speaking, in case that a single lens is held in a lens barrel, the lens barrel is so formed that the axis of the lens barrel is coincident with the centering axis of the lens. Accordingly, in case that a plurality of lenses are held in a lens barrel, the lens barrel must be so formed that the axis of the lens barrel is coincident with all the centering axes of the lenses. Generally, in case that the plurality of lenses are the axisymmetrical lenses, the centering axes of all the lenses can be positioned along one common axis; therefore, the plurality of lenses can be held within one lens barrel, no matter how many lenses are involved.
On the other hand, in case that one tries to constitute an optical system in which a decentered lens and an axisymmetrical lens are used, and particularly that the decentered lens is held in a lens barrel with its centering axis of the decentered lens being inclined with respect to the axis of the lens barrel, any favorable technique for holding the decentered lens relative to the lens barrel has not been established yet.
Although the lens having its centering axis which is necessary to be inclined with respect to the axis of the lens barrel can be a decentered lens, the lens is not necessarily a decentered lens, but can be an axisymmetrical lens.
On the other hands, in an attempt to apply such a conventional holding structure for axisymmetrical lenses as described above to holding a decentered lens, because the decentered lens is not uniform in thickness around its peripheral portion or its periphery, it is not possible to pinch or sandwich the decentered lens by using an annular member, as according to the conventional lens holding structure, which contacts the lens surface within one plane. Furthermore, because the decentered lens has a pair of different symmetrical axes for two lens surfaces thereof, it must be taken into consideration how the centering axis of the decentered lens should be set or determined in order that the lens is well assembled to the lens barrel. Such a technical problem has never been tackled up to now at a level for practical use of decentered lenses.
Under the circumstances that techniques dealing with decentered lens have not been established yet as mentioned above, a technical problem about how the positional relation between the centering axes of the lenses and the axis of the lens barrel should be set and determined relative to each other in order that two or more decentered lenses are effectively mounted on and well assembled into the lens barrel in case that they are combined with each other, could be a further complex problem.
Meanwhile, generally, in case that a lens is held in a lens barrel, the lens barrel is so constructed that the axis of the lens barrel is coincident with the centering axis of the lens. Accordingly, in case that a lens barrel holds a plurality of lenses, all the lenses must be so held in one lens barrel that all the lenses held therein have one common centering axis.
Generally, regarding lenses of the so-called axisymmetrical configuration in which two lens surfaces comprising spherical surfaces have one common axis, the centering axes of all the lenses can be made to coincide with only one common axis, no matter how many lenses are involved. Therefore, it is possible to hold the plurality of lenses within one lens barrel.
For an attempt to construct an optical system that includes a plurality of lenses, held in one lens barrel, at least one of which is a decentered lens, especially, for an attempt to construct the optical system in which the decentered lens is held therein with the centering axis of the decentered lens being inclined with respect to the axis of the lens barrel, a favorable technique for holding the decentered lens therein in such a manner has not been established yet.
Also, in an optical system made up with a plurality of lenses in which at least one decentered lens is incorporated, the optical arrangement can be such that centering axes are not coincident with each other between or among the decentered lenses or lens blocks including the decentered lenses.
However, a favorable technique has not been established yet for assembling a plurality of lens barrels (i.e. lens blocks), each of which holds a decentered lens or decentered lenses, to each other with the axes of the lens barrels being inclined relative to each other.
By the way, the lens to be held inside the lens barrel can be a decentered lens or decentered lenses; in other words, an axisymmetrical lens or axisymmetrical lenses can also be held therein.
Meanwhile, for the aforementioned axisymmetrical lens, a method of cutting an opening portion of the lens barrel by a plane vertical to the axis thereof in vicinity of the lens so as not to impair the protection of the lens surface, is commonly used. Accordingly, for the axisymmetrical lens, if the lens barrel is cut with the method, there does not arise such a problem as eclipsing the optical axis.
However, in case of constructing an optical system by incorporating a decentered lens in a lens barrel so that a centering axis of the decentered lens is determined with one of two lens surfaces thereof being as a reference surface, the centering axis of which is made to coincide with an axis of the lens barrel, pencils of light are eclipsed, or intercepted, by a part, not holding the decentered lens, of the lens barrel, if cutting the opening portion of the lens barrel by a plane perpendicular to the axis of the lens barrel, as cutting the opening portion of the lens barrel for an axisymmetrical lens.